Alan E. Fryar

Hydrogeochemical comparison and effects of overlapping redox zones on groundwater arsenic near the Western (Bhagirathi sub-basin, India) and Eastern (Meghna sub-basin, Bangladesh) margins of the Bengal Basin

Although arsenic (As) contamination of groundwater in the Bengal Basin has received wide attention over the past decade, comparative studies of hydrogeochemistry in geologically different sub-basins within the basin have been lacking. Groundwater samples were collected from sub-basins in the western margin (River Bhagirathi sub-basin, Nadia, India; 90 samples) and eastern margin (River Meghna sub-basin; Brahmanbaria, Bangladesh; 35 samples) of the Bengal Basin. Groundwater in the western site (Nadia) has mostly Ca-HCO(3) water while that in the eastern site (Brahmanbaria) is much more variable consisting of at least six different facies. The two sites show differences in major and minor solute trends indicating varying pathways of hydrogeochemical evolution However, both sites have similar reducing, postoxic environments (p(e): +5 to -2) with high concentrations of dissolved organic carbon, indicating dominantly metal-reducing processes and similarity in As mobilization mechanism. The trends of various redox-sensitive solutes (e.g. As, CH(4), Fe, Mn, NO(3)(-), NH(4)(+), SO(4)(2-)) indicate overlapping redox zones, leading to partial redox equilibrium conditions where As, once liberated from source minerals, would tend to remain in solution because of the complex interplay among the electron acceptors.

Nitrate reduction during ground-water recharge, Southern High Plains, Texas

Abstract In arid and semi-arid environments, artificial recharge or reuse of wastewater may be desirable for water conservation, but NO 3 − contamination of underlying aquifers can result. On the semi-arid Southern High Plains (USA), industrial wastewater, sewage, and feedlot runoff have been retained in dozens of playas, depressions that focus recharge to the regionally important High Plains (Ogallala) aquifer. Analyses of ground water, playa-basin core extracts, and soil gas in an 860-km 2 area of Texas suggest that reduction during recharge limits NO 3 − loading to ground water. Tritium and Cl − concentrations in ground water corroborate prior findings of focused recharge through playas and ditches. Typical δ 15 N values in ground water (>12.5‰) and correlations between δ 15 N and ln C NO − 3 –N suggest denitrification, but O 2 concentrations ≥3.24 mg l −1 indicate that NO 3 − reduction in ground water is unlikely. The presence of denitrifying and NO 3 − -respiring bacteria in cores, typical soil–gas δ 15 N values 3 − –N/Cl − and SO 4 2− /Cl − ratios with depth in cores suggest that reduction occurs in the upper vadose zone beneath playas. Reduction may occur beneath flooded playas or within anaerobic microsites beneath dry playas. However, NO 3 − –N concentrations in ground water can still exceed drinking-water standards, as observed in the vicinity of one playa that received wastewater. Therefore, continued ground-water monitoring in the vicinity of other such basins is warranted.

Controls on the regional-scale salinization of the Ogallala aquifer, Southern High Plains, Texas, USA

An extensive saline plume (>250 km 2 ) within the regionally important unconfined aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas field in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have d 18 O( ˇ6.7 toˇ8.8-) and d D( ˇ42 toˇ88-) values that tend to be depleted and have higher Cl (>150 mg/l) and SO4 (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters probably mixed with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer and the presence of potential pathways related to salt dissolution. # 2000 Elsevier Science Ltd. All rights reserved.

Hydraulic-conductivity reduction, reaction-front propagation, and preferential flow within a model reactive barrier

Recent work has demonstrated the utility of reactive barriers for transformation or immobilization of contaminants in ground water. However, reaction-induced changes in hydraulic conductivity may compromise the reactive barrier by diverting flow or focusing breakthrough of contaminants. For an acidic, ferric solution flowing through calcareous sand in column experiments, we investigated how hydraulic-conductivity reduction, reaction-front propagation, and fingering depend upon the reactive solid volume, reactive surface area, and pore-water velocity. Hydraulic-conductivity reductions are greater with larger initial pore-water velocity, calcite surface area, or calcite volume. Reductions in hydraulic conductivity (up to four orders of magnitude) result primarily from CO2 (g) exsolution rather than from ferric oxyhydroxide precipitation and may be at least partly reversed as bubbles migrate. Although fingering is both driven and repressed by pore-scale hydraulic changes, the velocity of the reaction front, width of the primary reaction zone, and maximum length of the largest finger appear to be insensitive to macroscopic changes in hydraulic conductivity at a constant flow rate. Reaction-front velocity increases as the ratio of initial calcite volume to pore-water velocity decreases, whereas zonal width and maximum finger length appear to increase as the Damkohler number decreases for a given calcite grain-size distribution. These results offer guidelines for improving the efficiency of reactive barriers when the reaction rate equals or exceeds the rate of mass transfer.

Modeling regional salinization of the Ogallala aquifer, Southern High Plains, TX, USA

Two extensive plumes (combined area .1000 km 2 ) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to .500 mg l 21 . Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the modeled plume area is in the range of 10 24 ‐10 25 m 3 day 21 , and the TDS concentrations are typically .3000 mg l 21 . Regions of increased salinity, identified within the Whitehorse Group (evaporite unit) underlying the Ogallala aquifer, are controlled by the structure and thickness variations relative to the recharge areas. Distinct flow paths, on the order of tens of km to .100 km in length, and varying flow velocities indicate that the salinization of the Ogallala aquifer has been a slow, ongoing process and may represent circulation of waters recharged during Pleistocene or earlier times. On-going pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping. Because the distribution of saline ground water in this region of the Ogallala aquifer is heterogeneous, careful areal and vertical characterization is warranted prior to any well-field development. q 2000 Elsevier Science B.V. All rights reserved.

Modeling the removal of metals from groundwater by a reactive barrier: Experimental results

The development of reactive barrier systems to remove metals from groundwater requires an improved understanding of the behavior of reaction fronts and the coupling between fluid flow, solute transport, reaction, and hydraulic changes. We report the results of experiments in which a dilute Fe(ClO4)3 solution was pumped through columns packed with mixtures of quartz sand and crushed calcite. The deposition of ferric oxyhydroxide peaked at, but was not restricted to, the visible precipitate front, where the neutralization of acidity generated by hydrolysis was completed. Even beyond this reaction front, calcite dissolution and ferric oxyhydroxide precipitation continued gradually. Upgradient from the front, where all calcite had been removed from the porous medium, ferric oxyhydroxide precipitation continued because of nucleation around older precipitate at a pH less than the influent pH. The velocity of the reaction front varied with the ratio of the initial volume of calcite to the pore water velocity, whereas the width of the zone of pronounced calcite dissolution varied as a function of the Damkohler number. These experiments illustrate how a model barrier can effectively and rapidly remove ferric iron from solution without fouling of the reactive surfaces.

Using Tracer Tests to Assess Natural Attenuation of Contaminants along a Channelized Coastal Plain Stream

Tracer tests have been widely used in studies of solute transport, gas exchange, and nutrient cycling in streams. However, the use of tracer tests to assess natural attenuation of ground-water-derived contaminants in streams, particularly from point sources, has been limited. We used tracer tests in conjunction with stream gauging and contaminant analyses to study the fate of trichloroethene (TCE) and technetium-99 (99Tc), which seep from industrial contaminant plumes into a channelized, first-order stream in the Coastal Plain of western Kentucky. Six tests were conducted over a 20-month period along a 300-m reach downstream of contaminated springs. Bromide, rhodamine WT, and nitrate were introduced as slug tracers to assess dilution, sorption, and reduction, respectively. Propane was added as a continuous, volatile tracer. Tracer transport was modeled as one-dimensional, with transient storage and first-order mass loss. Results indicate that (1) TCE is attenuated by volatilization; (2) TCE sorption, TCE reduction, and 99Tc reduction are negligible or absent; and (3) dilution is negligible along the study reach.

Composite use of numerical groundwater flow modeling and geoinformatics techniques for monitoring Indus Basin aquifer, Pakistan

The integration of the Geographic Information System (GIS) with groundwater modeling and satellite remote sensing capabilities has provided an efficient way of analyzing and monitoring groundwater behavior and its associated land conditions. A 3-dimensional finite element model (Feflow) has been used for regional groundwater flow modeling of Upper Chaj Doab in Indus Basin, Pakistan. The approach of using GIS techniques that partially fulfill the data requirements and define the parameters of existing hydrologic models was adopted. The numerical groundwater flow model is developed to configure the groundwater equipotential surface, hydraulic head gradient, and estimation of the groundwater budget of the aquifer. GIS is used for spatial database development, integration with a remote sensing, and numerical groundwater flow modeling capabilities. The thematic layers of soils, land use, hydrology, infrastructure, and climate were developed using GIS. The Arcview GIS software is used as additive tool to develop supportive data for numerical groundwater flow modeling and integration and presentation of image processing and modeling results. The groundwater flow model was calibrated to simulate future changes in piezometric heads from the period 2006 to 2020. Different scenarios were developed to study the impact of extreme climatic conditions (drought/flood) and variable groundwater abstraction on the regional groundwater system. The model results indicated a significant response in watertable due to external influential factors. The developed model provides an effective tool for evaluating better management options for monitoring future groundwater development in the study area.

Controls on ground water chemistry in the central Couloir Sud Rifain, Morocco.

Irrigation, urbanization, and drought pose challenges for the sustainable use of ground water in the central Couloir sud rifain, a major agricultural region in north-central Morocco, which includes the cities of Fès and Meknès. The central Couloir is underlain by unconfined and confined carbonate aquifers that have suffered declines in hydraulic head and reductions in spring flow in recent decades. Previous studies have surveyed ground water flow and water quality in wells and springs but have not comprehensively addressed the chemistry of the regional aquifer system. Using graphical techniques and saturation index calculations, we infer that major ion chemistry is controlled (1) in the surficial aquifer by cation exchange, calcite dissolution, mixing with deep ground water, and possibly calcite precipitation and (2) in the confined aquifer and warm springs by calcite dissolution, dolomite dissolution, mixing with water that has dissolved gypsum and halite, and calcite precipitation. Analyses of (2)H and (18)O indicate that shallow ground water is affected by evaporation during recharge (either of infiltrating precipitation or return flow), whereas deep ground water is sustained by meteoric recharge with little evaporation. Mechanisms of recharge and hydrochemical evolution are broadly consistent with those delineated for similar regional aquifer systems elsewhere in Morocco and in southern Spain.

Groundwater Flow and Reservoir Management in a Tributary Watershed along Kentucky Lake

ABSTRACT Understanding groundwater flow in tributary watersheds is important for evaluating water and solute storage and inputs into reservoirs. We delineated groundwater flow at various spatial and temporal scales within the watershed of Ledbetter Creek, a third-order tributary of the Tennessee River (impounded to create Kentucky Lake) in western Kentucky. We monitored hydraulic heads in wells (primarily in the upper watershed) and piezometers (in the lower watershed) and measured the flow of a spring along the embayment where the creek enters the reservoir. Manual measurements were made at least quarterly from July 1999 to March 2002 and were made annually each April from 2002 through 2006. From May 2000 to March 2002, hydraulic heads were recorded continuously in selected piezometers. At the watershed scale, groundwater flow followed the topography, with discharge occurring along the creek and in the embayment. Hydraulic heads in piezometers responded to large storms over periods of hours to days. Longer-term fluctuations in hydraulic head reflect reservoir management in the embayment (stage increased in early spring and decreased in late summer) and seasonal variability elsewhere in the watershed.